ISO 8601 prescribes, as a minimum, a four-digit year [YYYY] to avoid the year 2000 problem.It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD.[YYYY] indicates the ISO week-numbering year which is slightly different from the traditional Gregorian calendar year (see below).
However, years prior to 1583 are not automatically allowed by the standard.
Instead "values in the range  through  shall only be used by mutual agreement of the partners in information interchange." Calendar date representations are in the form shown in the adjacent box.
Earlier dates, in the proleptic Gregorian calendar, may be used by mutual agreement of the partners exchanging information.
The standard states that every date must be consecutive, so usage of the Julian calendar would be contrary to the standard (because at the switchover date, the dates would not be consecutive).
If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year (there is no week 00).
28 December is always in the last week of its year.
In addition, dates and times to be represented cannot include words with no specified numerical meaning in the standard (e.g., names of years in the Chinese calendar) or that do not use characters (e.g., images, sounds).
In representations for interchange, dates and times are arranged so the largest temporal term (the year) is placed to the left and each successively smaller term is placed to the right of the previous term.
It unified and replaced a number of older ISO standards on various aspects of date and time notation: ISO 2014, ISO 2015, ISO 2711, ISO 3307, and ISO 4031.
ISO 8601 is currently in the process of being updated and split into two parts anticipated to be released in 2018.
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